It has long been known to test electrical circuits either to establish their continuity, or to discover open circuits, short circuits, or other mistakes that electricians may have made in connecting the wires. Many kinds of apparatus have been used for these purposes.
One type of manufactured cable is the so-called "satin cable", which is flat and has all its wires in a side-by-side relationship and encased in a cover material. While short circuits, open circuits, or grounds can still occur in this type of cable, the configuration of this cable as manufactured makes it essentially impossible to mix up wire pairs inside the cable, and also reduces the likelihood of mixing up wiring connections at the cable ends.
The twisted pair circuit enjoys an advantage for alternating current operation in that currents passing through it will have decreased interaction with the surrounding environment. Thus in telephone circuitry, for example, twisted-pair circuits that occupy a common housing have far less mutual interference or "cross-talk" between circuits than if only straight wires were used.
The present invention relates to cables having a plurality of twisted-pair circuits bundled together in a common package. In addition to the possibility of short circuits or open circuits, the use of this type of cable also involves a risk of mixing up wiring connections at the cable ends, or of mispairing the wires inside the cable even if they were correctly connected at the cable ends. When multiple circuits are present, the problem of complete and accurate testing is complicated and time-consuming.
Correctly assembling and installing twisted pair cable is a difficult task. The right-hand wire and the left-hand wire of a pair change places every time the wires are twisted by 180 degrees. Even with good techniques for color coding of the wires it is still relatively easy to make a mistake in connecting to the contacts or terminals at the cable ends. The wires may be connected to correct contacts or terminals at the ends of the cable, but nevertheless may have been mis-paired inside the cable, where the error is difficult to detect. The electrician may connect the wire ends in exactly the same manner at each end of the cable, but may have misunderstood the plan by which the wires were to be connected, so there may still be mis-paired wires inside the cable.
Heavy demands are made upon circuitry when wiring that would in earlier times have carried only voice signals is now required to carry data signals at binary digit rates of 100,000 or more bits per second. A very slight degradation in circuit performance can quite significantly increase the probability of error in data transmission.
While standard methods of testing have been very effective for most purposes, one of the most subtle and difficult problems is cross-talk or mutual interference between adjacent circuits that appear to be correctly wired, but in fact are not. Specifically, the following scenario may sometimes arise.
A cable containing four wires forming two separate two-wire circuits is connected at each of its ends to terminals or contacts A, B, C, and D. The wires of one wire pair are connected to the A and B terminals at each end of the cable, while the wires of the other wire pair are connected to the C and D terminals. The tests made for continuity, short circuits, open circuits, or grounds, disclose no problem. However, inside the cable package, wires A and C are twisted together and wires B and D are twisted together. In other words, the wires are mis-paired inside the cable. In actual operation of the circuits to transmit data signals, the cross-talk or mutual interference between the A-B circuit and the C-D circuit will therefore be intolerable.